Low-molecular-weight heparin

In clinical situations in which the antithrombotic effect of LMWHs needs to be neutralized, protamine is used to neutralize heparin by binding to it.[9] Studies in animals and in vitro studies have demonstrated that protamine neutralizes the antithrombin activity of LMWHs, normalizing the aPTT and thrombin time. However, protamine appears to only partially neutralize the anti-factor Xa activity of LMWH. Because the molecular weight of heparin impacts its interaction with protamine, it is likely that the lack of complete neutralization of anti-factor Xa is due to a reduced protamine binding to the LMWHs moieties in the preparation. Protamine is a medicine that requires a high level of caution when used.

LMWH trials usually excluded individuals with unpredictable pharmacokinetics, and as a result patients with risks such as the severely obese or in advanced stages of kidney failure show decreased benefits due to fractionated heparin's increased half-life.[14] LMWHs should be used with extreme caution in patients undergoing any procedure involving spinal anaesthesia/puncture, in conditions with increased risk of bleeding or in patients with a history of heparin-induced thrombocytopenia.

Coagulation cascade is a normal physiological process which aims at preventing significant blood loss or hemorrhage following vascular injury. Unfortunately, there are times when a blood clot (thrombus) will form when it is not needed. For instance, some high risk conditions such as prolonged immobilization, surgery, or cancer can increase the risk of developing a blood clot which can potentially lead to significant consequences.

The coagulation cascade consists of a series of steps in which a protease cleaves and subsequently activates the next protease in the sequence.[2] Since each protease can activate several molecules of the next protease in the series, this biological cascade is amplified. The result of these reactions is to convert fibrinogen, a soluble protein, to insoluble threads of fibrin. Together with platelets, the fibrin threads form a stable blood clot.

Antithrombin (AT), a serine protease inhibitor, is the major plasma inhibitor of coagulation proteases.[15] LMWHs inhibit the coagulation process through binding to AT via a pentasaccharide sequence. This binding leads to a conformational change of AT which increases the rate at which it inhibits activated factor X (factor Xa). Once dissociated, the LMWH is free to bind to another antithrombin molecule and subsequently inhibit more activated factor X. Unlike AT activated by heparin, AT activated by LMWH cannot inhibit thrombin (factor IIa), but can only inhibit clotting factor Xa.

The effects of LMWHs cannot be acceptably measured using the partial thromboplastin time (PTT) or activated clotting time (ACT) tests.[16] Rather, LMWH therapy is monitored by the anti-factor Xa assay, measuring anti-factor Xa activity rather than a clotting time. The methodology of an anti-factor Xa assay is that patient plasma is added to a known amount of excess recombinant factor X and excess antithrombin. If heparin or LMWH is present in the patient plasma, it will bind to antithrombin and form a complex with factor X, inhibiting it from becoming factor Xa.[17] The amount of residual factor Xa is inversely proportional to the amount of heparin/LMWH in the plasma. The amount of residual factor Xa is detected by adding a chromogenic substrate that mimics the natural substrate of factor Xa, making residual factor Xa cleave it, releasing a colored compound that can be detected by a spectrophotometer.[17] Antithrombin deficiencies in the patient do not affect the assay, because excess amounts of antithrombin is provided in the reaction.[17] Results are given in units/mL of antifactor Xa, such that high values indicate high levels of anticoagulation and low values indicate low levels of anticoagulation in the plasma sample.[17]

LMWHs have a potency of greater than 70 units/mg of anti-factor Xa activity and a ratio of anti-factor Xa activity to anti-thrombin activity of >1.5.[18] (see table 1)

Table 1 Molecular weight (MW) data and anticoagulant activities of currently available LMWH products. Adapted from Gray E et al. 2008.[19]

Comparisons between LMWHs prepared by similar processes vary. For example, a comparison of Dalteparin and Nadroparin suggests they are more similar than products produced by different processes. However, comparison of enoxaparin and tinzaparin shows they are very different from each other with respect to chemical, physical, and biological properties.

As might be expected, products prepared by distinctly different processes are dissimilar in physical, chemical, and biological properties.[2][15] Hence a slight change in the depolymerisation process could result in substantial variation of the structure or composition of a given LMWH.

Therefore, for every LMWH, a strictly defined depolymerisation procedure is needed to guarantee the sameness of the final LMWH product and the predictability of clinical outcomes. LMWHs, as biological origin products, rely on stringent manufacturing procedures to guarantee the absence of biological or chemical contamination. It is therefore critical to adopt stringent manufacturing practices, through rigorous quality assurance steps, to ensure the highest quality of the produced LMWHs and to guarantee patient safety. These quality assurance steps, to be effective, need to be implemented from the raw material (crude heparin) collection to the final LMWH product.

Due to these identified and potential differences, several organizations, including the United States Food and Drug Administration, the European Medicines Agency, and the World Health Organization, regard LMWHs as individual products that should not be considered as clinically equivalent, as they differ in many crucial aspects such as molecular, structural, physiochemical, and biological properties.[21][22][23] According to international guidelines, the choice of an individual LMWH should be based on its proven clinical safety and efficacy for each indication.[13] Therefore, switching from LMWH to another LMWH during treatment is not recommended during clinical practice.[24]

Differences from heparin (i.e. "unfractionated heparin") include:

• Average molecular weight: heparin is about 15 kDa and LMWH is about 4.5 kDa.[25]
• Less frequent subcutaneous dosing than for heparin for postoperative prophylaxis of venous thromboembolism.
• Once or twice daily subcutaneous injection for treatment of venous thromboembolism and in unstable angina instead of intravenous infusion of high dose heparin.
• No need for monitoring of the APTT coagulation parameter as required for high dose heparin.[26]
• Possibly a smaller risk of bleeding.
• Smaller risk of osteoporosis in long-term use.
• Smaller risk of heparin-induced thrombocytopenia, a potential side effect of heparin.
• The anticoagulant effects of heparin are typically reversible with protamine sulfate, while protamine's effect on LMWH is limited.
• LMWH has less of an effect on thrombin compared to heparin, but about the same effect on Factor Xa.
• Due to its renal clearance, LMWH is contraindicated in patients with kidney disease in whom unfractionated heparin can be used safely.

When the commercial patent of a LMWH expires, a generic or biosimilar LMWH can then be marketed. The first 'generic' LMWH was approved by the Food and Drug Administration in July 2010. The FDA has used 5 analytical and pharmacological criteria to establish the authenticity of a generic LMWH, without requiring clinical studies in patients.[27]

From a regulatory viewpoint, the FDA considers LMWHs (as well as insulin, glucagon and somatropin) as "generic" drugs, even though they may be sourced from biological material. The European Medicines Agency considers LMWHs as biologicals so their regulatory approval - as biosimilars - is approached differently compared to the FDA.[28][29]